Can an aftermarket stereo be wired up to the standard 6 disc changer in my bravo?

I reckon it'd depend on the HU. Some HUs will only fit with the same manufacturers changers, e.g. JVC HU and JVC changer. I've got a feeling the majority of HUs will not compatible with the Fiat changer..................

IIRC its a goodmans (fiat)HU as standard in the bravo so perhaps one of those but lets be honest you want kenwood(my brand of choice)so get a kenwood cd changer or perhaps a music keg

Even if you get the same brand headunit as ur cd changer it may not be compatible. Some car manufacturers insist on making their audio equipment "different" so you have to go back to them if you upgrade. You might get lucky though